Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231207 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 807 AM EDT Mon Oct 23 2017 .SYNOPSIS... A potent upper level trough and accompanying surface frontal system will approach from the west and cross the Appalachians and middle Atlantic states this afternoon through tonight. A secondary cold front will follow and cross our region late Tue and Tue night. && .NEAR TERM /THROUGH TONIGHT/... As of 415 AM Monday... WV imagery this morning indicates a potent closed mid level low/shortwave trough has begun to assume a negative tilt from srn MO/nrn AR sewd into srn MS. This feature, and associated preceding 50-100 m mid level height falls over the srn middle Atlantic states this aft-eve, is forecast to lift and accelerate newd through the OH valley and srn-cntl Appalachians through early tonight, before becoming absorbed by early Tue by a larger-scale trough/developing closed mid-upper low amplifying from s-cntl Canada to the Great Lakes. At the surface, a 1010 mb surface low now over sern IL will rapidly deepen while migrating nnewd, to around 980 mb by the time it reaches the vicinity of nrn MI 24-30 hours from now. A trailing cold front now stretching swd from that low, through w-cntl TN and AL, will sweep ewd into the wrn Carolinas by this evening, then into the ern Carolinas by Tue morning. Secondary cyclogenesis now underway over nrn AL will continue to occur at the intersection of this cold front and a warm front extending sewd from this secondary low, around the rim of the srn Appalachians/n-cntl GA enewd to nern SC/sern NC. This secondary low will migrate newd through the wrn Carolinas, along the retreating warm front and what will likely become an in-situ wedge front as early day showers overspread the wrn Carolinas and interact with a lingering continental air mass from the high pressure ridge that has been in place in recent days. An axis of low clouds and showers now stretching across SC will pivot newd across the srn and wrn NC Piedmont this morning; and this should serve to hold temperatures aob 70 degrees over the wrn Piedmont today, and lead to the development of the aforementioned in- situ wedge front. Meanwhile, multi-layered cloud cover will increase across the remainder of cntl NC, but with peaks of at least filtered sunshine that should allow temperatures to climb into middle 70s to around 80 degrees - similar to those of recent days. Strong and focused forcing for ascent accompanying both the negatively-tilted trough aloft and the surface front will likely result in a strongly-forced QLCS, possibly preceded by a warm frontal band of scattered convection, which will spread east across the srn/wrn Piedmont between 4-8 PM, then ewd across the Triangle and surrounding areas between 6-10 PM, and lastly across the I-95 corridor/Coastal Plain between 9 PM and 1 AM. Instability will be maximized over the ern Carolinas, into perhaps the 500-1000 J/kg range, while the corridor of strongest forcing for ascent will translate through the Piedmont of the Carolinas and VA, where weaker instability on the order of several hundred J/kg will likely materialize. Though weaker, this buoyancy should still be sufficient to support a severe threat owing to at least moist- neutral lapse rates in the lowest km, and more than sufficient lift to compensate for the relative minimum in instability. The degree and character of both low and deep layer shear will be more than supportive of organized storm modes, including supercells and bows/lewps --with isolated tornadoes-- embedded within what will otherwise be a strongly-forced QLCS that will pose a risk of strong to damaging wind gusts along its length, resulting from momentum transport of a 50-70 kt llj between 925-850 mb. That llj will also contribute to extreme low level shear characterized by enlarged/cyclonically-curved hodographs and associated 400-800 m2/s2 of effective SRH, strongest over the Piedmont, where the risk of the aforementioned isolated tornadoes appears to be greatest. Lastly, storm total rainfall amounts are expected to range from around one half to one and a half inches, highest over the wrn Piedmont, where both early day showers and the stronger forcing for ascent will favor the highest amounts. While these values are well short of 1-3 hr flash flood guidance values in the 3-4" range, some urban flooding will be possible, particularly around the Triad. Clearing behind the convective line, with the passage of the cold front, will spread west to east overnight. Areas of low clouds and fog may redevelop over the srn/wrn piedmont, where clearing will occur first, and lessening winds between this lead cold front, and a trailing one scheduled for a late Tue-Tue night passage, will favor modest-strong radiational cooling. Lows are expected to range from the upper 40s west to lower-mid 60s east. && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... As of 310 AM Monday... Tuesday morning, precipitation associated with a cold front moving through central NC will be in the vicinity of the eastern edges of the CWA and heading eastward with time. Since the low pressure system associated with this front will be located well to the northwest of the area over the Great Lakes, it will take some time for the colder airmass to filter in behind the front. Therefore high temperatures will remain fairly high on Tuesday with upper 60s to mid 70s expected across the area from NW to SE. Lows Tuesday night will be in the low 40s across the NW Piedmont to near 50 degrees in eastern counties. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 310 AM Monday... After the surface front moves through the area by Tuesday morning, attention will turn to the parent low pressure system which will hang back over the Great Lakes for a day or so before weakening considerably and heading due north into Canada. As a result of this, the upper level trough never really digs southward and upper level flow over central NC doesn`t ever go northwesterly. While the surface air will certainly be cooler than we have been, it isn`t an extremely cold airmass that will be filtering into the area. Therefore expect highs to drop into the upper 50s to mid 60s Wednesday and Thursday but then moderate once again to near 70 degrees for the end of the work week. The coldest night will be Wednesday night but guidance shows only upper 30s to lower 40s and with some light westerly winds and possibly some clouds aloft, frost may not be a threat. Lows will remain in the 40s for the majority of the week and then climb back into the upper 40s to lower 50s for the weekend. The next frontal system will approach the area next weekend but there is uncertainty with regards to timing and also the evolution of a low pressure system riding up the southeast coastline prior to the arrival of the cold front that could cause a chance of precipitation earlier in the weekend. && .AVIATION /12Z MONDAY THROUGH SATURDAY/... As of 800 AM Monday... In strengthening and increasingly moist east to sely low level flow off the Atlantic ocean, an axis of IFR-MVFR ceilings and showers will continue to move across wrn portions of the forecast area/mainly at Triad TAF sites this morning, then continue, with periods of IFR ceilings, today through this evening. Aside from a brief instance of an IFR-MVFR ceiling and/or a shower at RDU/FAY/RWI this morning, generally VFR conditions are expected to persist at those locations until this evening. An approaching frontal system will then result in the ewd progression of a band of IFR-MVFR ceilings, heavy showers and isolated storms with strong and gusty sly to sswly winds --including some severe with isolated tornadoes and straight line wind gusts aoa 50 kts-- between 22Z/23rd and 05Z/24th. Sely surface winds will otherwise increase and become strong and gusty by late morning to midday, ahead of the expected line(s) of convection. West to east clearing will occur with the passage of the associated cold front this evening-early tonight, with a period of enhanced swly winds in the hour or three immediately following the frontal passage. Lessening winds and lingering low level moisture late tonight may result in the development of fog and low stratus primarily at INT and GSO between 08-12Z/24th, though the latest model guidance has backed off on this potential. Outlook: VFR conditions are anticipated for at least the next several days. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...Ellis LONG TERM...Ellis AVIATION...MWS

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